Intra- and interspecific relationships between nest size and immunity

Nest-building behavior has been suggested to represent a postmating sexually selected signal in passerine birds, an hypothesis that has received both comparative and experimental support. Because selection pressure due to parasites and diseases should be particularly high during nest building, mainly due to energetic costs and depression of the immune system associated with this reproductive phase, we predicted a positive association between nest-building effort and immunity. Nest-building effort would reflect the ability to produce efficient immune responses of builders only if individuals with a superior immune system would display exaggerated nest-building effort. We tested this prediction by studying the relationship between volume of nest material used for nest construction and, at the intraspecific level, estimates of innate humoral immune response in barn swallows Hirundo rustica. At the interspecific level, we used responses to the mitogenic phytohemagglutinin as an indicator of adaptive immune response of European passerine species. As predicted, we found, after controlling for several potential confounding factors, that volume of nest material was positively related to immune response both at the intra- and at the interspecific level. Alternative hypotheses explaining the comparative results are discussed. Copyright 2007, Oxford University Press.